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+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CRC_INTERNAL_CRC_H_
+#define ABSL_CRC_INTERNAL_CRC_H_
+
+#include <cstdint>
+
+#include "absl/base/config.h"
+
+// This class implements CRCs (aka Rabin Fingerprints).
+// Treats the input as a polynomial with coefficients in Z(2),
+// and finds the remainder when divided by an primitive polynomial
+// of the appropriate length.
+
+// A polynomial is represented by the bit pattern formed by its coefficients,
+// but with the highest order bit not stored.
+// The highest degree coefficient is stored in the lowest numbered bit
+// in the lowest addressed byte.   Thus, in what follows, the highest degree
+// coefficient that is stored is in the low order bit of "lo" or "*lo".
+
+// Hardware acceleration is used when available.
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace crc_internal {
+
+class CRC {
+ public:
+  virtual ~CRC();
+
+  // Place the CRC of the empty string in "*crc"
+  virtual void Empty(uint32_t* crc) const = 0;
+
+  // If "*crc" is the CRC of bytestring A, place the CRC of
+  // the bytestring formed from the concatenation of A and the "length"
+  // bytes at "bytes" into "*crc".
+  virtual void Extend(uint32_t* crc, const void* bytes,
+                      size_t length) const = 0;
+
+  // Equivalent to Extend(crc, bytes, length) where "bytes"
+  // points to an array of "length" zero bytes.
+  virtual void ExtendByZeroes(uint32_t* crc, size_t length) const = 0;
+
+  // Inverse opration of ExtendByZeroes.  If `crc` is the CRC value of a string
+  // ending in `length` zero bytes, this returns a CRC value of that string
+  // with those zero bytes removed.
+  virtual void UnextendByZeroes(uint32_t* crc, size_t length) const = 0;
+
+  // If *px is the CRC (as defined by *crc) of some string X,
+  // and y is the CRC of some string Y that is ylen bytes long, set
+  // *px to the CRC of the concatenation of X followed by Y.
+  virtual void Concat(uint32_t* px, uint32_t y, size_t ylen);
+
+  // Apply a non-linear transformation to "*crc" so that
+  // it is safe to CRC the result with the same polynomial without
+  // any reduction of error-detection ability in the outer CRC.
+  // Unscramble() performs the inverse transformation.
+  // It is strongly recommended that CRCs be scrambled before storage or
+  // transmission, and unscrambled at the other end before futher manipulation.
+  virtual void Scramble(uint32_t* crc) const = 0;
+  virtual void Unscramble(uint32_t* crc) const = 0;
+
+  // Crc32c() returns the singleton implementation of CRC for the CRC32C
+  // polynomial.  Returns a handle that MUST NOT be destroyed with delete.
+  static CRC* Crc32c();
+
+ protected:
+  CRC();  // Clients may not call constructor; use Crc32c() instead.
+
+ private:
+  CRC(const CRC&) = delete;
+  CRC& operator=(const CRC&) = delete;
+};
+
+}  // namespace crc_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CRC_INTERNAL_CRC_H_